1. Field of the Invention
This invention relates to a general purpose photoelectric conversion device having one or more cells, and particularly to such a device for use in an image input stage of a facsimile apparatus.
2. Prior Art
A recent trend in the art of facsimile apparatus design involves a contact type image sensor provided with a reading device, having a long dimension equal to the width of manuscript paper. This type of sensor is employed in an image input stage of a facsimile apparatus and has the advantage that it can be miniaturized and can avoid the use of a reducing optical system.
This contact type image sensor employs an amorphous silicon hydride (a-Si:H) semiconductor as a photoelectric conversion layer. Photoelectric conversion cells having a sandwich structure, in which a photoelectric conversion layer is disposed between metal and transparent electrodes, are arranged in the image sensor of this type. Thus, this contact type image sensor has a simple structure and good photoelectric conversion characteristics.
FIG. 5 shows a cross-sectional view of a conventional sensor. In this figure, reference numeral 1 denotes an insulating substrate; 2 a metal electrode; 3 a semiconductor layer; 4 a transparent electrode; 5 a transparent insulating film; 5a an open portion of the transparent insulating film; and 6 wiring metal.
In the conventional photoelectric conversion device, a metal electrode film 2, of approximately 1000 .ANG. thickness, made of a metal such as chromium is formed on a glass substrate 1 by using a sputtering technique. By going through a patterning process, the film is formed into the metal electrode 2 as a common electrode and shaped like a band, typically spanning all of the reading cells of an array, which array would extend linearly in the vertical direction in FIG. 1(a).
Then, by performing a plasma-assisted chemical vapor deposition (CVD) technique, the photo-responsive semiconductor layer 3 made of amorphous silicon hydride, of 1 .mu.m film thickness, is formed by using silane (SiH.sub.4) gas on condition that the flow rate thereof is 20-50 SCCM; the pressure thereof 0.2-0.5 Torr; the temperature of the substrate 150.degree.-250.degree. C.; RF power 10-50 mW/cm.sup.2 ; and elapsed time 30-60 minutes.
Subsequently, a transparent electrode layer 4 of 800 .ANG. thickness is formed, by using DC magnetron sputtering, of tin indium oxide. Furthermore, a resist is coated on this layer and then, by going through a patterning process employing a photoetching method, a transparent electrode 4 is formed for each individual reading cell.
The semiconductor layer 3 is etched using this patterned resist as a mask, by a mixed gas composed of tetrafluoromethane and oxygen, so that an individual semiconductor layer 3 is formed for each cell. After the resist is removed, a transparent insulating film 5 is formed by a coating technique; and the open portion 5a for connecting the wiring metal 6 thereto is made by a patterning technique. Each separate photoelectric conversion cell (reading element) is made by connecting the wiring metal 6 to the transparent electrode 4 through the open portion 5a (see, for example, Japanese Patent Application Provisional Publication No. 63-7772 Official Gazette).
One might expect this photoelectric conversion cell, made using the above described process, to operate as follows: When light is irradiated on the semiconductor layer 3 with the bias voltage being applied thereto through the metal electrode 2 and wiring metal 6, carriers are produced in the semiconductor layer 3 and a photoelectric current (I.sub.p) is drawn through the wiring metal 6 and metal electrode 2. In contrast, when light is not irradiated thereon, the quantity of dark current should be small because a Schottky barrier exists between the semiconductor 3 and the transparent electrode 4 even with the bias voltage applied. Thus, one might expect the photoelectric conversion cell to function as an efficient photosensor.
Nevertheless, the sequence of processes for fabricating this sensor includes a process of depositing a film of metal 6, by sputtering. Because this process involves the impact of ions on the surface of the transparent electrode layer 4, aluminum atoms penetrate into the transparent electrode layer, composed of tin indium oxide, or equivalent, and further into the semiconductor layer 3 composed of amorphous silicon hydride. As a result, the Schottky barrier is insufficiently formed; and, instead, a contact similar to an ohmic contact is formed. Further, the quantity of the dark current becomes larger in proportion to the area of the open 5a. Thus, the contrast ratio (the ratio of photoresponsive current to dark current) is insufficient for the cell to function as an efficient sensor.